Optical surface preservation techniques and apparatus

ABSTRACT

Techniques and architecture are disclosed for preserving optical surfaces (e.g., windows, coatings, etc.) in a flowing gas amplifier laser system, such as a diode-pumped alkali laser (DPAL) system. In some instances, the disclosed techniques/architecture can be used, for example, to protect optical surfaces in a DPAL system from: (1) chemical attack by pump-bleached alkali vapor atoms and/or ions; and/or (2) fouling by adherence thereto of reaction products/soot produced in the DPAL. Also, in some instances, the disclosed techniques/architecture can be used to substantially match the geometry of the pumping volume with that of the lasing volume, thereby minimizing or otherwise reducing the effects of amplified spontaneous emission (ASE) on DPAL output power. Furthermore, in some cases, the disclosed techniques/architecture can be used to provide a DPAL system capable of producing a beam output power in the range of about 20 kW to 10 MW, or greater.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.14/063,727, filed Oct. 25, 2013, which claims the benefit of U.S.Provisional Application No. 61/718,909, filed on Oct. 26, 2012, and U.S.Provisional Application No. 61/730,443, filed on Nov. 27, 2012, theentire contents of all of which are herein incorporated by reference.

FIELD OF THE DISCLOSURE

This disclosure relates to laser systems and more particularly toflowing gas amplifier laser systems.

BACKGROUND

Laser systems involve a number of non-trivial challenges, and lasersystems which utilize gaseous lasing media have faced particularcomplications, such as those with respect to preserving optical surfacesimplemented therein.

SUMMARY

One example embodiment of the present invention provides a diode-pumpedalkali laser (DPAL) including an optical surface, a flow of a lasinggas, wherein the lasing gas includes alkali vapor, and a flow of a firstnon-alkali gas flowing between the optical surface and the flow of thelasing gas. In some cases, the flow of the first non-alkali gas forms aprotective layer along the optical surface to prevent or minimize atleast one of chemical attack thereto and/or soot buildup thereon. Insome instances, the first non-alkali gas comprises at least one of aninert gas, a noble gas, a hydrocarbon, a fluorocarbon, and/or acombination of any thereof. In some cases, the DPAL further includes aflow of a second non-alkali gas flowing between the optical surface andthe flow of the first non-alkali gas. In some such cases, the flow ofthe second non-alkali gas forms a protective layer along the opticalsurface to prevent or minimize at least one of chemical attack theretoand/or soot buildup thereon. In some other such cases, the secondnon-alkali gas comprises at least one of an inert gas, a noble gas, ahydrocarbon, a fluorocarbon, and/or a combination of any thereof. In anyof the aforementioned cases, the alkali vapor can comprise at least oneof rubidium (Rb), cesium (Cs), and/or potassium (K). In any of theaforementioned cases, the optical surface can comprise an opticalwindow. In any of the aforementioned cases, the optical surface cancomprise a mirror. In some such cases, the optical surface furthercomprises an optical coating.

Another example embodiment of the present invention provides a systemincluding a diode-pumped alkali laser (DPAL) having an optical pumpingcavity and a diffusive/convective end window assembly optically coupledwith the optical pumping cavity, the assembly including a tube having afirst end, a second end located opposite the first end, and a channeldefined between the first and second ends, wherein the first end isconfigured to be optically coupled with the optical pumping cavity, anoptical window at the second end of the tube, and one or more gasinjectors operatively coupled to the tube for delivering a flow of anon-alkali gas to the channel. In some cases, the flow of the non-alkaligas forms a protective layer along the optical window to prevent orminimize at least one of chemical attack thereto and/or soot buildupthereon. In some cases, the flow of the non-alkali gas traverses alength of the channel in about 0.1-5 seconds. In some instances, thesystem further includes a gas circulation system fluidly coupled withthe diffusive/convective end window assembly and configured to deliverthe flow of the non-alkali gas to the one or more gas injectors. In someinstances, the optical window comprises at least one of fused silicaand/or sapphire. In some example cases, the optical window isperpendicular relative to the channel. In some other example cases, theoptical window is angled at Brewster's angle relative to the channel. Insome instances, the DPAL utilizes at least one of a rubidium (Rb)-based,cesium (Cs)-based, and/or potassium (K)-based lasing gas. In some suchinstances, the non-alkali gas is the same as a carrier gas for thelasing gas. In any of the aforementioned cases, the non-alkali gas cancomprise at least one of an inert gas, a noble gas, a hydrocarbon, afluorocarbon, and/or a combination of any thereof. In any of theaforementioned cases, the DPAL can have an output power in the range ofabout 20 kW to 10 MW. In any of the aforementioned cases, the system canbe operatively coupled with at least one of a land vehicle, awatercraft, an aircraft, a spacecraft, a building, and/or a bunker.

Another example embodiment of the present invention provides a systemincluding a diode-pumped alkali laser (DPAL) having an optical pumpingcavity and a side window assembly optically coupled with the opticalpumping cavity, the assembly including a recess including an opticalwindow and one or more gas injectors operatively coupled to the recessfor delivering a flow of a non-alkali gas to the recess. In some cases,the flow of the non-alkali gas forms a protective layer along theoptical window to prevent or minimize at least one of chemical attackthereto and/or soot buildup thereon. In some cases, the flow of thenon-alkali gas has a flow rate in the range of about 0.1-5 cm/s. In someinstances, the system further includes a gas circulation system fluidlycoupled with the side window assembly and configured to deliver the flowof the non-alkali gas to the one or more gas injectors. In some cases,the optical window comprises at least one of fused silica and/orsapphire. In some instances, the DPAL utilizes at least one of arubidium (Rb)-based, cesium (Cs)-based, and/or potassium (K)-basedlasing gas. In some such instances, the non-alkali gas is the same as acarrier gas for the lasing gas. In any of the aforementioned cases, thenon-alkali gas can comprise at least one of an inert gas, a noble gas, ahydrocarbon, a fluorocarbon, and/or a combination of any thereof. In anyof the aforementioned cases, the DPAL can have an output power in therange of about 20 kW to 10 MW. In any of the aforementioned cases, thesystem can be operatively coupled with at least one of a land vehicle, awatercraft, an aircraft, a spacecraft, a building, and/or a bunker.

Another example embodiment of the present invention provides a systemincluding a diode-pumped alkali laser (DPAL) having an optical pumpingcavity and a flow assembly optically coupled with the optical pumpingcavity, the flow assembly including a first conduit having a nozzleportion, a diffuser portion, and a gap there between, wherein the firstconduit is configured to deliver a flow of a lasing gas to the opticalpumping cavity, and a second conduit surrounding the first conduit,wherein the second conduit is configured to deliver a flow of a firstnon-alkali gas to the optical pumping cavity, wherein the flow of thefirst non-alkali gas circumscribes the flow of the lasing gas. In somecases, the second conduit has an optical window disposed therein, andthe flow of the first non-alkali gas forms a protective layer along theoptical window to prevent or minimize at least one of chemical attackthereto and/or soot buildup thereon. In some such cases, the opticalwindow comprises at least one of fused silica and/or sapphire. In someinstances, the first non-alkali gas comprises at least one of an inertgas, a noble gas, a hydrocarbon, a fluorocarbon, and/or a combination ofany thereof. In some instances, the system further includes a gascirculation system fluidly coupled with the flow assembly and configuredto deliver the flow of the lasing gas to the first conduit and the flowof the first non-alkali gas to the second conduit. In some cases, thesystem further includes a housing surrounding the second and firstconduits, wherein the housing has an optical window disposed therein andis configured to deliver a flow of a second non-alkali gas whichsurrounds the flow of the first non-alkali gas, and wherein the flow ofthe second non-alkali gas forms a protective layer along the opticalwindow to prevent or minimize at least one of chemical attack theretoand/or soot buildup thereon. In some such cases, the optical windowcomprises at least one of fused silica and/or sapphire. In some suchcases, the second non-alkali gas comprises at least one of an inert gas,a noble gas, a hydrocarbon, a fluorocarbon, and/or a combination of anythereof. In some such instances, the system further includes a gascirculation system fluidly coupled with the flow assembly and configuredto deliver the flow of the lasing gas to the first conduit, the flow ofthe first non-alkali gas to the second conduit, and the flow of thesecond non-alkali gas to the housing. In some of the aforementionedcases, the optical pumping cavity can include a stable opticalresonator. In some other of the aforementioned cases, the opticalpumping cavity can include an unstable optical resonator. In any of theaforementioned cases, the nozzle portion can include an exit throughwhich the flow of the lasing gas passes to enter the optical pumpingcavity. In some such cases, the exit of the nozzle portion has at leastone dimension in the range of about 1-10 mm. In some other such cases,the exit of the nozzle portion has at least one dimension in the rangeof about 1-10 cm. In any of the aforementioned cases, the flow of thelasing gas can have a flow velocity in the range of about 50-1000 m/s.In any of the aforementioned cases, the flow of the lasing gas and theflow of the first non-alkali gas can have approximately equal flowvelocities. In any of the aforementioned cases, the DPAL can utilize atleast one of a rubidium (Rb)-based, cesium (Cs)-based, and/or potassium(K)-based lasing gas. In some such instances, the first non-alkali gasis the same as a carrier gas for the lasing gas. In any of theaforementioned cases, the DPAL can have an output power in the range ofabout 20 kW to 10 MW. In any of the aforementioned cases, the system canbe operatively coupled with at least one of a land vehicle, awatercraft, an aircraft, a spacecraft, a building, and/or a bunker.

The features and advantages described herein are not all-inclusive and,in particular, many additional features and advantages will be apparentto one of ordinary skill in the art in view of the drawings,specification, and claims. Moreover, it should be noted that thelanguage used in the specification has been selected principally forreadability and instructional purposes and not to limit the scope of theinventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a damaged window of a cesium (Cs) cell of an examplediode-pumped alkali laser (DPAL) system.

FIG. 2A represents an example side-pumped DPAL lasing cavity.

FIG. 2B represents an example end-pumped DPAL lasing cavity.

FIG. 3 is a side cross-sectional view of an end window assemblyconfigured in accordance with an embodiment of the present invention.

FIG. 4A is a side cross-sectional view of an example DPAL pumping cavityhaving a recessed side window.

FIG. 4B is a side cross-sectional view of a recessed side windowassembly configured in accordance with an embodiment of the presentinvention.

FIG. 5A is a side perspective view of a flush side window assemblyconfigured in accordance with an embodiment of the present invention.

FIG. 5B is a side perspective view of an inner conduit of the flush sidewindow assembly of FIG. 5A, in accordance with an embodiment of thepresent invention.

FIG. 6A is a partial cross-sectional view of a flush side windowassembly configured in accordance with an embodiment of the presentinvention.

FIG. 6B is a partial cross-sectional view of a flush side windowassembly configured in accordance with an embodiment of the presentinvention.

FIG. 7A is a side perspective view of a multiple barrier side windowassembly configured in accordance with an embodiment of the presentinvention.

FIG. 7B is a partial cross-sectional view of a multiple barrier sidewindow assembly configured in accordance with an embodiment of thepresent invention.

FIG. 8A is a schematic view of an example implementation of a lasing gascirculation system configured in accordance with an embodiment of thepresent invention.

FIG. 8B is a schematic view of an example implementation of a lasing gascirculation system configured in accordance with an embodiment of thepresent invention.

FIG. 9 is a schematic view of an example implementation of a gascirculation system configured in accordance with an embodiment of thepresent invention.

These and other features of the present embodiments will be understoodbetter by reading the following detailed description, taken togetherwith the figures herein described. The accompanying drawings are notintended to be drawn to scale. In the drawings, each identical or nearlyidentical component that is illustrated in various figures isrepresented by a like numeral. For purposes of clarity, not everycomponent may be labeled in every drawing.

DETAILED DESCRIPTION

Techniques and architecture are disclosed for preserving opticalsurfaces (e.g., windows, coatings, etc.) in a flowing gas amplifierlaser system, such as a diode-pumped alkali laser (DPAL) system. In someinstances, the disclosed techniques/architecture can be used, forexample, to protect optical surfaces in a DPAL system from: (1) chemicalattack by pump-bleached alkali vapor atoms and/or ions; and/or (2)fouling by adherence thereto of reaction products/soot produced in theDPAL. Also, in some instances, the disclosed techniques/architecture canbe used to substantially match the geometry of the pumping volume withthat of the lasing volume, thereby minimizing or otherwise reducing theeffects of amplified spontaneous emission (ASE) on DPAL output power.Furthermore, in some cases, the disclosed techniques/architecture can beused to provide a DPAL system capable of producing a beam output powerin the range of about 20 kW to 10 MW, or greater. Numerousconfigurations and variations will be apparent in light of thisdisclosure.

General Overview

As previously indicated, there are a number of non-trivial issues thatcan arise which complicate laser systems which utilize gaseous lasingmedia. For instance, in the example context of alkali vapor lasersystems (e.g., such as a diode-pumped alkali laser, or DPAL, system),one non-trivial issue pertains to the fact that alkali atoms (e.g.,rubidium, cesium, potassium, etc.) present in the gain medium may beoptically pumped into one or more excited states and/or become ionized,for example, by the pump laser and/or by the output beam and in turncause damage to the optical surfaces (e.g., windows, coatings, etc.)with which they come in contact (e.g., attack the surface coatings ofthe optical windows where the pump and output laser beams pass through).Another example complication is that, within DPAL systems which utilizehydrocarbons in the gas mixture, reaction products (soot) may begenerated from chemical reactions within the DPAL lasing cavity. Thesesoot particles can adhere to windows and other optical surfaces, absorbenergy, and degrade the coatings thereon (e.g., localized heating maycause etching).

As will be appreciated in light of this disclosure, these examplecomplications can occur with any of a wide range of gas amplifier mediaand regardless of whether the DPAL is flowing or static, end-pumped orside-pumped. To demonstrate this point, consider FIG. 1, for example,which illustrates a damaged window of a cesium (Cs) cell of an exampleDPAL system after operation thereof at temperatures in excess of 120° C.As will be further appreciated in light of this disclosure, attempts atincreasing the output power of existing DPAL systems, for example, toachieve continuous-wave (CW) operation at power output levels higherthan 10 W will worsen the damage done to the DPAL optical components(e.g., windows, coatings, etc.), substantially decreasing the servicelifetime of such components and thus negatively impacting the ability toachieve such CW operation.

Yet another non-trivial issue pertains to the fact that, when thepopulation inversion in a given regional sub-volume of a DPAL system isjust above threshold, alkali atoms which have been pumped by the pumplaser but which have not been stimulated immediately to cause emission(i.e., which do not lie within a regional sub-volume that is illuminatedwithin an intense mode, such as the fundamental transverse mode (TEM₀₀mode), of the output beam of such system) may spontaneously emitphotons, which can lead to amplified spontaneous emission (ASE). ASEsaps power from the DPAL output beam and generates a significant amountof heat, consequently reducing the overall optical-to-optical efficiencyof the system and imposing constraints on the output power achievablewith a given DPAL system. These and other non-trivial issues preventexisting DPAL designs/approaches from achieving high-power, continuousoperation.

Some laser systems (e.g., a DPAL) may utilize a lasing gas which isflowed, for example, to provide for a desired amount of heat removal fora given target application. For instance, a flowing lasing gas may beutilized in: (1) transversely pumped (e.g., side-pumped) DPAL lasingcavities, such as the example shown in FIG. 2A, which may be pumped fromone or multiple sides; and/or (2) end-pumped DPAL lasing cavities, suchas the example shown in FIG. 2B, which may be pumped from one ormultiple ends. As will be appreciated in light of this disclosure, andin accordance with an embodiment, the fluid motion of the lasing gasprovides an opportunity for use thereof in establishing and/ormaintaining a DPAL lasing medium and removing heat therefrom whileprotecting/preserving the optical integrity of various optical surfaces(e.g., windows, coatings, etc.) which may be present in a given DPAL.

Thus, and in accordance with one set of embodiments, techniques andarchitecture are disclosed for preserving optical surfaces in a lasersystem which utilizes a flowing gaseous lasing medium, such as adiode-pumped alkali laser (DPAL) system. The disclosedtechniques/architecture can be used, in accordance with an embodiment,in side-pumped (e.g., transversely pumped from one or more sidesthereof) and/or end-pumped DPALs (e.g., pumped from one or more endsthereof). Also, the disclosed techniques/architecture can be used, inaccordance with an embodiment, in a DPAL system which utilizes, forexample, a flowing alkali-based lasing gas, such as, but not limited to:(1) a rubidium (Rb)-based lasing gas; (2) a cesium (Cs)-based lasinggas; and/or (3) a potassium (K)-based lasing gas. Other suitable usesand/or laser system configurations for the disclosedtechniques/architecture will be apparent in light of this disclosure.

In accordance with an embodiment, the disclosed techniques/architecturecan be used to provide a flowing fluid barrier layer which issubstantially free of alkali vapor (e.g., a non-alkali gaseous barrier)and which isolates, in part or in whole, the lasing medium (e.g., alkalivapor-saturated lasing gas) from the optical surfaces (e.g., windows,coatings, etc.) of a given DPAL. The disclosed techniques/architecturecan be used, in accordance with an embodiment, to protect opticalsurfaces in a DPAL system (or other laser system which utilizes aflowing gaseous amplifier medium) from: (1) chemical attack bypump-bleached alkali vapor atoms and/or ions; and/or (2) fouling byadherence thereto of reaction products/soot produced in the DPAL.

In accordance with an embodiment, the gaseous barrier layer may beprovided, for example: (1) as a bleed flow which prevents or otherwisereduces migration of the lasing gas to a given DPAL optical surface;and/or (2) as a co-flowing stream/jet which circumscribes/envelops, andthus confines, the lasing gas, preventing or otherwise reducingmigration of the lasing gas to a given DPAL optical surface. In someinstances, and in accordance with an embodiment, a bleed flow of barriergas and a co-flowing stream/jet of barrier gas may be provided/utilizedsimultaneously within a given DPAL system. In some embodiments, such agaseous barrier layer can be provided in a continually replenishedmanner and, as such, may be substantially indestructible and/ornon-fouling, thus allowing for indefinitely preserving the opticalquality of a given optical surface. In some cases, and in accordancewith an embodiment, a solid barrier layer (e.g., a hard coating on anoptical window or other surface) may be implemented in conjunction witha gaseous barrier layer provided as described herein, for example, tofurther assist with protecting a given DPAL optical surface.

As will be appreciated in light of this disclosure, and in accordancewith an embodiment, it may be desirable to ensure that the one or moregases which constitute the gaseous barrier layer are substantiallytransparent (e.g., fully or otherwise within an acceptable tolerance) tominimally or otherwise negligibly interfere with the optical pumping(e.g., by a pump laser or other suitable source) of the lasing gas. Aswill be further appreciated, it may be desirable to ensure that the flowof the gaseous barrier layer (e.g., bleed flow and/or co-flow) minimallyor otherwise negligibly: (1) disturbs the pumping region; and/or (2)results in refractive lensing of the DPAL output beam. Still further, itmay be desirable to provide a gaseous barrier layer which has beenconditioned, for example, to minimize or otherwise reduce temperaturevariations therein.

The disclosed techniques/architecture may be compatible with a widevariety of optical surfaces. For example, some embodiments can be usedwith DPAL systems that implement fused silica and/or sapphire windows.Furthermore, some embodiments can be used with DPAL optics which areprovided with one or more optical and/or protective coatings (e.g.,high-transmission coatings; high-reflectivity coatings; nanotexturedsurfaces; etc.).

Also, the disclosed techniques/architecture may be compatible with awide variety of gases which may be used in the non-alkali gaseousbarrier layer. Some example suitable gases include, but are not limitedto: (1) inert gases (e.g., elemental gas, compound gas, etc.); (2) noblegases, such as helium (He) and/or isotopes thereof, such as helium-4(⁴He), helium-3 (³He), etc.; (3) hydrocarbons, such as methane (CH₄),ethane (C₂H₆), etc.; (4) fluorocarbons; (5) a combination of any of theaforementioned; and/or (6) any other suitable gas or gaseous mixturewhich can provide the desired thermal management performance and/orassist with the desired spin-orbit atomic transitions for the lasinggas, as will be apparent in light of this disclosure.

Furthermore, the disclosed techniques/architecture may be compatiblewith a wide variety of optical pumping sources and/or methods, inaccordance with an embodiment. For instance, some embodiments of thepresent invention may be compatible with optical pumping provided by oneor more laser diodes (e.g., laser diode bars, stacks, arrays, etc.).Some embodiments may be compatible with adjustments to the pump beamgeometry (e.g., converging pump beam; adjusted Gaussian waist diameter;etc.). Also, some embodiments may be compatible with any of a wide rangeof optical pumping wavelengths (e.g., 780 nm, etc.) and/or spectrallinewidths.

As discussed below, some embodiments of the present invention mayrealize improved output power, for example, by reducing efficiencylosses in power from amplified spontaneous emission (ASE), therebyimproving the optical-to-optical efficiency of a given DPAL system. Insome example cases, optical-to-optical efficiency may be improved to bein the range of about 40-80% or greater (e.g., about 40% or greater;about 50% or greater; about 60% or greater; about 70% or greater; about80% or greater; etc.). In some such instances, a sufficiently highoptical-to-optical efficiency may be achieved, for example, to provide aDPAL system which is capable of having a beam output power in the rangeof about 20 kW to 10 MW, or greater. Other achievable performancecharacteristics/ranges will be apparent in light of this disclosure.

As will be appreciated in light of this disclosure, some embodiments ofthe present invention may realize benefits/advantages as compared withexisting approaches/designs. For example, as previously noted, someembodiments of the present invention may realize an increase in theservice lifetime of a given DPAL optical component as compared withexisting approaches/designs. Also, some embodiments may eliminate orotherwise reduce the need to implement, in a given DPAL, opticalsurfaces (e.g., windows, coatings, etc.) which have been specificallydesigned, for instance, to tolerate sustained exposure to alkali vaporwhich has been excited by high laser fluxes (e.g., nano-texturedsapphire).

As will be further appreciated in light of this disclosure, someembodiments of the present invention may be used to provide a DPALsystem having optical componentry which exhibits an improved/enhancedservice lifetime and which is compatible with high-power (e.g.,kilowatt- and/or megawatt-class), continuous-wave (CW) DPAL operation.Such a DPAL system may be utilized, in accordance with one or moreembodiments, in any of a wide variety of applications, such as, but notlimited to: (1) welding and metal cutting; (2) mining; (3) medicalprocedures; (4) directed energy weapons (DEWs) and countermeasures; (5)deorbiting space debris; and/or (6) any other directed energyapplication (e.g., transmitting energy to a deep-space probe). In someinstances, and in accordance with an embodiment, a DPAL system providedusing the disclosed techniques/architecture can be configured, forexample, to be deployed on a vehicle (e.g., land vehicle, watercraft,aircraft, spacecraft, etc.), an infrastructure (e.g., building, bunker,etc.), and/or any other desired platform, temporary or permanent. Othersuitable uses will be apparent in light of this disclosure.

As used herein, the term “lasing gas” generally may refer to a gas orgas mixture including, for example: (1) a vapor of alkali metal, such asrubidium (Rb), cesium (Cs), potassium (K), etc., having a density in therange of about 1×10¹¹/cm³ to 1×10¹⁶/cm³; (2) a buffer gas whichpressure-broadens the absorption lines of the alkali atom with absolutepressure in the range of about 150-15,000 torr; and/or (3) a gas toaccelerate the atomic spin-orbit transition from the pump level to thelasing level, which may be the aforementioned buffer gas or one or moreother gases. Also, as used herein, the term “non-alkali gas” generallymay refer to a gas or gas mixture including, for example, a buffer gasand/or an atomic spin-orbit transition gas (like those discussed aboveregarding the lasing gas), but which is devoid of alkali vapor or whichhas a minimal or otherwise negligible amount of alkali vapor (e.g., thealkali vapor has been removed to as low a level as is practical orotherwise desirable). It should be noted, however, that the claimedinvention is not intended to be limited to only these example lasinggases and non-alkali gases. Rather, any gas or gas mixture havingqualities described herein may be suitable to serve as a lasing gasand/or non-alkali gas. Numerous configurations will be apparent in lightof this disclosure.

Bleed Flow Techniques and End Window Assembly

As previously noted, and in accordance with an embodiment, the disclosedtechniques/architecture can be used to provide a bleed flow of anon-alkali gas (or gas mixture) which provides a gaseous barrier layer.In some cases, such a bleed flow can be used, for example, tocontinuously purge a given window or other optical surface of a DPAL (orother laser system which utilizes a flowing gaseous amplifier medium),thereby providing a diffusive/convective gaseous barrier layer whichprevents or otherwise reduces migration of lasing gas to thatwindow/surface. As discussed below, such a gaseous barrier layer may beimplemented, for example, to protect a given DPAL lasing cavity endwindow. As will be appreciated in light of this disclosure, thetechniques disclosed herein for protecting a given end window may beutilized, in some embodiments, regardless of: (1) how the DPAL is pumped(e.g., end pumped from one or more ends, side pumped from one or moresides, etc.); (2) whether a co-flow (e.g., such as that discussed belowwith reference to FIGS. 5A-5B and 6A-6B) is present; and/or (3) endwindow orientation (e.g., perpendicular to the lasing axis, offset at anangle relative to the lasing axis such as the Brewster's angle, etc.).

In some cases, and in accordance with an embodiment, a DPAL system (orother laser system which utilizes a flowing gaseous amplifier medium)may be provided with structure/architecture configured to assist withproviding such a diffusive/convective gaseous barrier layer. Forinstance, consider FIG. 3, which is a side cross-sectional view of anend window assembly 100 configured in accordance with an embodiment ofthe present invention. As can be seen from the depicted exampleembodiment, end window assembly 100 may include a tube 110 which extendsout from the pumping cavity 10 of the DPAL and which may have a channel116 defined therein. In some cases, an end window 120 can be disposed atan end of tube 110, and the DPAL output beam may be directedtherethrough. In some embodiments, end window 120 may be oriented suchthat its major surfaces (e.g., the surfaces through which the DPALoutput beam is to pass) are substantially perpendicular (e.g., exactlyor otherwise within a desired variance) to the lasing axis of the DPAL.However, the claimed invention is not so limited, as in some otherembodiments, end window 120 may be oriented/offset at an angle relativeto the lasing axis (e.g., at or near the Brewster's angle; etc.). Also,tube 110 may be provided with one or more injectors 130 which deliverthe gases to one or more locations for providing the gaseous barrierlayer to the channel 116 of tube 110. In accordance with an embodiment,lasing cavity end window 120 may be isolated from the alkali-saturatedlasing gas (e.g., lasing medium containing Rb vapor, Cs vapor, K vapor,etc.) utilized in the DPAL by virtue of how end window assembly 100provides a bleed flow of a barrier gas which opposes diffusion of thelasing gas into the channel 116 of tube 110. As will be appreciated inlight of this disclosure, end window assembly 100 may includeadditional, fewer, and/or different elements or components from thosehere described, and the claimed invention is not intended to be limitedto any particular assembly configurations, but can be used with numerousconfigurations in numerous applications.

In accordance with an embodiment, the geometry of tube 110 may becustomized for a given target application. For instance, in someembodiments, tube 110 may be configured as a substantially hollowcylinder having one or more open ends (e.g., an entrance end 112 and anexit end 114). However, the claimed invention is not so limited, as insome other embodiments, tube 110 may be configured as a substantiallyhollow square/rectangular prism having one or more such open ends112/114. Other suitable geometries for tube 110 will depend on a givenapplication and will be apparent in light of this disclosure.

In accordance with an embodiment, tube 110 may be dimensioned (e.g.,length, width/diameter, etc.) as desired for a given target application.For instance, tube 110 may be provided, in some example embodiments,with a length and/or transverse dimension (width/diameter) in the rangeof about 1 mm to 10 cm or greater. As will be appreciated in light ofthis disclosure, it may be desirable to ensure that the length of tube110 is sufficient to limit the ability of the lasing gas and/or sootparticles to migrate from the pumping cavity 10 down the length of tube110 to end window 120. As will be further appreciated, it may bedesirable to ensure that the width/diameter of tube 110 is sufficient toallow passage therethrough of the DPAL output beam (e.g., the transversedimension of tube 110 is greater than the transverse dimension of theDPAL output beam).

In some cases, tube 110 may be configured such that itslength-to-width/diameter ratio is sufficiently high (e.g., about 5-to-1or higher), for example, to allow for the use of high lasing gas flowrates (e.g., in the range of about 5-20 cm/s or higher). As will beappreciated in light of this disclosure, and in accordance with anembodiment, such a high ratio may minimize or otherwise reduce anylikelihood of the lasing gas and/or soot particles migrating down thelength of tube 110 to reach window 120, regardless of any turbulencewhich might result at entrance end 112 from the high lasing gas flowrate. Thus, in some such instances, the flow rate of the barrier gasbleed flow may be reduced with minimal or otherwise negligible risk ofdamage to end window 120. However, the claimed invention is not solimited, as tube 110 may be configured, in some other cases, with lowerlength-to-width/diameter ratios (e.g., less than about 5-to-1). Itshould be noted, however, that with some lower ratios (e.g., about2-to-1 or lower), it may be desirable to ensure that: (1) the flow rateof the barrier gas bleed flow is not reduced excessively; and/or (2) anaerodynamic adjustment 140 (discussed below) for turbulence in pumpingregion 10 is made. Other suitable dimensions (e.g., lengths,widths/diameters, ratios, etc.) for tube 110 will depend on a givenapplication and will be apparent in light of this disclosure.

In accordance with an embodiment, the geometry of channel 116 may becustomized for a given target application. For instance, in someembodiments, channel 116 may be configured with a substantiallyrectilinear geometry. However, the claimed invention is not so limited,as in some other embodiments, channel 116 may be provided with asubstantially curvilinear geometry. As will be appreciated in light ofthis disclosure, and in accordance with an embodiment, the geometry ofchannel 116 may depend, in part or in whole, on the geometry and/ordimensions of the DPAL output beam. Other suitable geometries forchannel 116 will depend on a given application and will be apparent inlight of this disclosure.

In accordance with an embodiment, the dimensions (e.g., length,width/diameter, etc.) of channel 116 may be customized for a giventarget application. For instance, in some embodiments, channel 116 maybe provided with a length that substantially matches that of tube 110(e.g., channel 116 extends from entrance end 112 to exit end 114). Insome cases, channel 116 may be configured such that one portion thereofis longer than another portion thereof (e.g., as discussed below in thecontext of aerodynamic adjustment 140). Also, in some embodiments,channel 116 may be provided with a width/diameter that is sufficient(e.g., just sufficient or otherwise within a given tolerance) to allowpassage therethrough of the DPAL output beam. For example, channel 116may have a width/diameter, in some cases, in the range of about 1 mm to5 cm or greater. As will be appreciated in light of this disclosure, andin accordance with an embodiment, provision of a narrow channel 116(e.g., having a width/diameter in the range of about 1-5 mm) may bedesirable, for example, for relatively low to modest power applications,while provision of a wider channel 116 (e.g., having a width/diameter inthe range of about 1-5 cm or greater) may be desirable for relativelyhigher power applications. Other suitable dimensions (e.g., lengths,widths/diameters, etc.) for channel 116 will depend on a givenapplication and will be apparent in light of this disclosure.

As previously noted, end window assembly 100 may include one or moreinjectors 130 configured to deliver one or more gases to provide thegaseous barrier layer in the interior (e.g., channel 116) of tube 110.As can be seen from FIG. 3, the one or more injectors 116 may beoperatively coupled with tube 110 proximate exit end 114 thereof. Byvirtue of such placement, the barrier gas bleed flow may be deliveredwithin channel 116 proximate end window 120 and thus, in accordance withan embodiment, may help to minimize or otherwise reduce the ability ofthe lasing gas atoms/molecules and/or soot particles to migrate from thepumping cavity 10 down the length of tube 110 to contact end window 120.

In accordance with an embodiment, upon exiting the one or more injectors130, the barrier gas may flow through channel 116 towards pumping cavity10. During its transit through channel 116, the barrier gas may passthrough a transition region 118. Transition region 118 may define orotherwise provide an interface for the volume of the barrier gas and thevolume of the lasing gas. The dimensions and/or relative location oftransition region 118 within channel 116 may be made or otherwisepermitted to fluctuate depending on, for example: (1) the depth ofpenetration of the lasing gas flow into channel 116 from the entranceend 112, which may depend on the pressure, velocity, and/or laminarityof the lasing flow, the dimensions of channel 116, and/or thediffusivity of the lasing gas (e.g., depending on gas composition and/ortemperature); and/or (2) the flow rate and/or laminarity of the barriergas bleed flow.

Thereafter, the barrier gas bleed flow may enter a mixing region 119where it may be mixed with the lasing gas (and/or a co-flowing barriergas, discussed below) streaming past entrance end 112. Upon mixing withgas from the pumping cavity 10, the barrier gas may be carrieddownstream (e.g., in the direction generally indicated by the largearrows in FIG. 3). It may be desirable to configure mixing region 119such that the barrier gas bleed flow exiting entrance end 112 minimally(or otherwise negligibly) disrupts/displaces the lasing gas flow. Tothat end, and in accordance with an embodiment, computational fluiddynamics software/programming (e.g., ANSYS® Fluent® software) may beused, for example, to determine what geometry of mixing region 119minimizes or otherwise reduces the disruptive effects of the barrier gasbleed flow on the lasing gas flow and/or buffer gas co-flow in pumpingcavity 10.

In some cases, end window assembly 100 may include an aerodynamicadjustment 140. For example, as can be seen from FIG. 3, in someinstances, tube 110 may be configured such that the downstream edgethereof at entrance end 112 is laterally offset from the upstream edgethereof and widened/stepped outward relative to pumping cavity 10. Inaccordance with an embodiment, such aerodynamic adjustment 140 may help,for example, to accommodate the increase in the flow of gas passingthrough pumping cavity 10 which may result by virtue of the mixed gasbeing carried downstream. Aerodynamic adjustment 140 may permit themixed gas to expand downstream of the pumping cavity 10, thus allowingthe mixed gas to transition to a wall jet which may be effectivelyinjected parallel to the lasing gas flow, thereby minimally or otherwisenegligibly disrupting/displacing the lasing gas flow. In some cases,aerodynamic adjustment 140 may assist, for example, with maintaining thedesired transition region 118.

In some cases, it may be desirable to ensure that the temperature oftube 110 is held within a desired range for a given target application.For example, in some cases, it may be desirable to maintain thetemperature of tube 110 at room temperature (e.g., to maintain a lowvapor pressure of the alkali-saturated lasing gas). Other suitabletemperature ranges will depend on a given application and will beapparent in light of this disclosure.

In accordance with an embodiment, it may be desirable to ensure that thebarrier gas bleed flow has a sufficiently high flow rate to counter thediffusive flow of the lasing gas and/or to provide a sharp interfacialregion (e.g., transition region 118) between the lasing gas volume andthe barrier gas volume. However, it also may be desirable to ensure thatthe flow rate of the barrier gas is not so high that the pumping region10 experiences flow-induced unsteadiness (e.g., which may undesirablydisrupt/displace the lasing gas flow) as a result of the combining ofthe barrier gas with the lasing gas flow. Otherwise stated, if the flowrate of the barrier gas bleed flow is too high, mixing region 119 may bepushed outside of channel 116 and into pumping region 10, which mayinduce turbulence, alter alkali concentration, and/or negatively impactlaser performance, whereas if the flow rate is too low, the lasing gasmay penetrate too deeply into channel 116, pushing transition region 118closer to end window 120, and thus increasing the likelihood of damageto end window 120. Therefore, in one specific example embodiment,provision of a barrier gas bleed flow having a flow rate in the range ofabout 0.1-5 cm/s (e.g., about 1 cm/s or less) may be sufficient toprovide a substantially stationary transition region 118 having a length(parallel to the lasing axis) comparable to its transverse dimension(width/diameter perpendicular to the lasing axis). Other suitablevelocity ranges will depend on a given application and will be apparentin light of this disclosure.

In some embodiments, end window assembly 100 may be operatively coupled(e.g., fluidly coupled) with a gas circulation system 700 a, discussedbelow with reference to FIG. 8A, which is configured to provide thebarrier gas bleed flow for injection into end window assembly 100.

Bleed Flow Techniques and Recessed Side Windows

FIG. 4A is a side cross-sectional view of an example DPAL pumping cavityhaving a recessed side pumping window. When the lasing gas stream flowsacross the opening of the recess/cavity adjacent the side pumpingwindow, a recirculation zone (generally represented by the curved,dotted line) fills the recess/cavity, resulting in a buildup of sootwhich contaminates the side pumping window and decreases systemperformance.

As previously noted, and in accordance with an embodiment, the disclosedtechniques/architecture can be used to provide a bleed flow of anon-alkali gas (or gas mixture) which provides a gaseous barrier layer.In some cases, such a bleed flow can be used, for example, to oppose aninternal rotating flow, thereby countering the formation of arecirculation zone like that described with reference to FIG. 4A andthus minimizing or otherwise reducing the accumulation of soot on arecessed side window. Also, in some cases, the barrier gas bleed flowmay minimize or otherwise reduce the ability of the lasing gas particlesto migrate from the pumping region to a given recessed side window. Aswill be appreciated in light of this disclosure, the techniquesdisclosed herein for protecting a given recessed side window may beutilized, in some embodiments, regardless of: (1) how the DPAL is pumped(e.g., end pumped from one or more ends, side pumped from one or moresides, etc.); and/or (2) whether a co-flow (e.g., such as that discussedbelow with reference to FIGS. 5A-5B and 6A-6B) is present.

In some cases, and in accordance with an embodiment, a DPAL system (orother laser system which utilizes a flowing gaseous amplifier medium)may be provided with structure/architecture configured to assist withproviding such a gaseous barrier layer. For instance, consider FIG. 4B,which is a side cross-sectional view of a recessed side window assembly200 configured in accordance with an embodiment of the presentinvention. As can be seen from the depicted example embodiment, recessedside window assembly 200 may include a cavity/recess 216 which extendsfrom the pumping region 10 of the DPAL, a side window 220 disposed at anend thereof, through which the one or more pump beams may be directedtowards the lasing gas flow, and one or more injectors 230 which delivera barrier gas bleed flow to the cavity/recess 216. In accordance with anembodiment, recessed side window 220 may be isolated from thealkali-saturated lasing gas (e.g., lasing medium containing Rb vapor, Csvapor, K vapor, etc.) utilized in the DPAL by virtue of how recessedside window assembly 200 provides a barrier gas bleed flow which opposesdiffusion and/or recirculation of the lasing gas into cavity/recess 216.As will be appreciated in light of this disclosure, recessed side windowassembly 200 may include additional, fewer, and/or different elements orcomponents from those here described, and the claimed invention is notintended to be limited to any particular assembly configurations, butcan be used with numerous configurations in numerous applications.

In accordance with an embodiment, the geometry of cavity/recess 216 maybe customized for a given target application. For instance, in someembodiments, cavity/recess 216 may be configured with a substantiallysquare/rectangular geometry having one or more open ends (e.g., whichmay be configured, for example, to have a side window 220 disposedthereat). Furthermore, in accordance with an embodiment, the dimensions(e.g., length/depth, width/diameter, and/or height) of cavity/recess 216may be customized for a given target application. As will be appreciatedin light of this disclosure, it may be desirable to providecavity/recess 216 with a geometry and/or dimensions which do not resultin an overly complex external flow field which otherwise woulddisrupt/displace the lasing gas flow. As will be further appreciated, insome cases the depth-to-height ratio of cavity/recess 216 and/or theflow rate of the barrier gas bleed flow may be chosen, at least in part,based on the flow rate of the lasing gas, for example, to ensure thatturbulence in the mixing region prevents or otherwise minimizes theability of the alkali-saturated lasing gas and/or soot particles toreach side window 220. Other suitable geometries and/or dimensions forcavity/recess 216 will depend on a given application and will beapparent in light of this disclosure.

As previously noted, recessed side window assembly 200 may include oneor more injectors 230 configured to deliver the barrier gas bleed flowto the interior of cavity/recess 216. As can be seen from FIG. 4B, theone or more injectors 230 may be operatively coupled with the one ormore of the walls of cavity/recess 216. In some instances, anasymmetrical arrangement of injectors 230 may be provided, for example,to produce a desired vortex (discussed below) within cavity/recess 216.The quantity of injectors 230 utilized may depend, in some instances, onthe desired flow strength and/or cavity dimensions.

In accordance with an embodiment, the one or more injectors 230 maydeliver the barrier gas bleed flow into cavity/recess 216 so as togenerate an internal counter-rotating flow. If an asymmetricalarrangement of injectors 216 is used, then the asymmetrical nature ofthe barrier gas jet flow may produce an internal vortex that allows theflow across the cavity/recess 216 to substantially match that of thelasing gas flow. Thus, the lasing gas (and/or co-flow, when present) maybe prevented from entering the cavity/recess 216.

As will be appreciated in light of this disclosure, and in accordancewith an embodiment, the flow rate of the injected barrier gas can bevaried as desired for a given application. In some instances, it may bedesirable to produce an internal rotating flow having a velocity whichsubstantially matches that of the lasing gas stream (and/or co-flow,when present). As will be further appreciated, in some instances, it maybe desirable to match the velocities to minimize or otherwise reduceflow separation due to velocity mismatch. However, as similarly notedabove in the context of end window assembly 100, excessively increasingthe buffer gas bleed flow velocity may disrupt/displace the lasing gasflow outside of the desired pumping region, therebydiminishing/fluctuating the power of the DPAL output beam or otherwisedegrading system performance.

Also, much like in the context of the end window assembly 100 discussedabove, recessed side window assembly 200 may include an aerodynamicadjustment 240. For example, as can be seen from FIG. 4B, in someinstances cavity/recess 216 may be configured such that the downstreamedge thereof is laterally offset from the upstream edge thereof andwidened/stepped outward relative to pumping cavity 10. In accordancewith an embodiment, such aerodynamic adjustment 240 may help, forexample, to accommodate the increase in the flow of gas passing throughpumping cavity 10 which may result by virtue of the barrier gas bleedflow being carried downstream. Aerodynamic adjustment 240 may permit thebarrier gas to expand downstream of the pumping cavity 10, thus allowingthe injected barrier gas bleed flow to transition to a wall jet whichmay be effectively injected parallel to the lasing gas flow, therebyminimally or otherwise negligibly disrupting/displacing the lasing gasflow.

In some embodiments, recessed side window assembly 200 may beoperatively coupled (e.g., fluidly coupled) with a gas circulationsystem 700 b, discussed below with reference to FIG. 8B, which isconfigured to provide the barrier gas bleed flow for injection intorecessed side window assembly 200.

Co-Flow Techniques and Architecture

In a transversely pumped (e.g., side-pumped) DPAL, it may be desirable,in some instances, to ensure that the side windows are as close aspossible to the lasing gas (or otherwise within a given tolerance).Therefore, in some cases, windows which are substantially flush with thepumping cavity may be utilized. However, such a configuration may resultin the amplifier gas flow being substantially closer to the windowsurface as compared, for instance, to a recessed side window (discussedabove). As will be appreciated in light of this disclosure, thisproximity may increase the susceptibility of a given flush side windowto chemical attack and/or soot deposition.

As previously noted, and in accordance with an embodiment, the disclosedtechniques/architecture can be used to provide a co-flowing stream/jetof a non-alkali gas (or gas mixture) which substantially confines thelasing gas (e.g., by circumscribing or otherwise enveloping the lasinggas on all sides). As such, the co-flowing gas may serve as a gaseousbarrier layer which may flow substantially flush with the walls of thepumping cavity (and thus a given flush side window), thereby preventingor otherwise reducing migration of the lasing gas to such window in aDPAL. As will be appreciated in light of this disclosure, the techniquesdisclosed herein for protecting a given side window (e.g., with one ormultiple barrier gas layers) may be utilized, in some embodiments,regardless of: (1) how the DPAL is pumped (e.g., end pumped from one ormore ends, side pumped from one or more sides, etc.); and/or (2) whethera bleed flow (e.g., such as that discussed above with reference to FIGS.3 and 4B) is present. As will be further appreciated, some embodimentsalso may prevent or otherwise reduce migration of the lasing gas to theone or more end windows in a DPAL. Numerous configurations will beapparent in light of this disclosure.

In some cases, and in accordance with an embodiment, a DPAL system (orother laser system which utilizes a flowing gaseous amplifier medium)may be provided with structure/architecture configured to assist withproviding such a gaseous barrier layer. For instance, consider FIG. 5A,which is a side perspective view of a flush side window assembly 300configured in accordance with an embodiment of the present invention,and FIG. 5B, which is a side perspective view of an inner conduit 310 ofthe flush side window assembly 300 of FIG. 5A, in accordance with anembodiment of the present invention. As can be seen from these figures,flush side window assembly 300 may include an inner conduit 310 having anozzle portion 312 and a diffuser portion 314 and having a gap 316 therebetween (e.g., such that the walls defining inner conduit 310 areinterrupted or otherwise not continuous). As can be seen further, anouter conduit 360 may be provided around inner conduit 310 (e.g.,coaxially or as otherwise desired for a given application) and mayinclude one or more side windows 320 and/or end windows 322 disposedtherein. As will be appreciated in light of this disclosure, flush sidewindow assembly 300 may include additional, fewer, and/or differentelements or components from those here described, and the claimedinvention is not intended to be limited to any particular assemblyconfigurations, but can be used with numerous configurations in numerousapplications. For instance, in some cases, one or more of side windows320 and/or end windows 322 may be configured as an aperture/open spacerather than an optical window, and the one or more pump beams and/orDPAL output beam may be permitted to pass therethrough. In some cases,one or more of side windows 320 and/or end windows 322 may be configuredas an optical mirror rather than an optical window, and the one or morepump beams and/or DPAL output beam may be reflected, partially or fully,therefrom.

As discussed below, inner conduit 310 may be configured to deliver alasing gas flow to pumping cavity 10 while outer conduit 360 may beconfigured to deliver a co-flowing planar jet of a non-alkali barriergas (e.g., a gas or gas mixture that is Rb-free, Cs-free, K-free, etc.)which circumscribes the lasing gas volume and which flows flush with theone or more flush side windows 320. Downstream of pumping cavity 10,inner conduit 310 and outer conduit 360 may be configured to recapture,in part or in whole, the lasing gas and the co-flowing gas,respectively.

In some cases, outer conduit 360 may serve as the pressure vessel forthe gas flow of flush side window assembly 300. For instance, in somecases, outer conduit 360 may be configured such that flush side windowassembly 300 is capable of handling pressures in the range of about 1-20atm or greater (e.g., about 2-10 atm or greater). In some other cases,outer conduit 360 may serve as a vacuum vessel, for example, foreliminating air from the vessel and/or for guiding the gas flow (e.g.,barrier gas co-flow and lasing gas flow) through flush side windowassembly 300. As such, it may be desirable to provide an outer conduit360, for example, having substantially rigid walls. To that end, and inaccordance with an embodiment, outer conduit 360 may be constructed, inpart or in whole, for example, with: (1) a metal (e.g., titanium,aluminum, etc.); (2) an alloy (e.g., stainless steel); (3) a ceramic;(4) an optical material, such as fused silica, sapphire, etc.; (5) ahigh-temperature polymer (e.g., polytetrafluoroethylene); (6) acombination of any of the aforementioned; and/or (7) any other suitablematerial, as will be apparent in light of this disclosure.

In accordance with an embodiment, outer conduit 360 (and thus flush sidewindow assembly 300) may be generally configured with a contoured shape.For instance, as can be seen from FIG. 5A, the upstream portion 362 ofouter conduit 360 may be provided with a generally converging shape,while the downstream portion 364 thereof may be provided with agenerally diverging shape. In some cases, a throated portion 366 havinga substantially straight shape may be located between upstream portion362 and downstream portion 364. By virtue of such a configuration, outerconduit 360 (and thus flush side window assembly 300) may be configured,in general, to convert pressure to velocity (e.g., from the upstreamportion 362 to the throated portion 366) and then back to pressure again(e.g., from the throated portion 366 through the downstream portion364). The generally converging shape of upstream portion 362 of outerconduit 360 may help to accelerate the barrier gas co-flow, whereas thegenerally diverging shape of downstream portion 364 of outer conduit 360may help to decelerate the barrier gas co-flow. As discussed below,inner conduit 310 may be provided with a similar converging-divergingconfiguration and thus may provide similar pressure/velocitycapabilities, for example, in the context of the lasing gas flow. Aswill be appreciated in light of this disclosure, it may be desirable toconfigure outer conduit 360 so as to: (1) minimize or otherwise reducepower losses which may result during the pressure/velocity conversionprocesses; and/or (2) accelerate and/or decelerate the barrier gasco-flow with minimal or otherwise negligible loss of stagnationpressure.

In accordance with an embodiment, outer conduit 360 may be configured toprovide, in part or in whole, the flow path for the barrier gas co-flow.For instance, the interstitial space between the walls of outer conduit360 and the walls of inner conduit 310 (discussed below) generally maydefine the flow path for the barrier gas co-flow, in accordance with anembodiment.

As can be seen from FIG. 5A, in some embodiments, outer conduit 360 mayhave disposed therein one or more optical windows (and/or mirrors,apertures, etc.) which may be used, for example, in a DPAL system. Forinstance, in some cases, outer conduit 360 may have one or more flushside windows 320 disposed therein. In some such embodiments, the flushside window(s) 320 may be disposed at throated portion 366 andsubstantially aligned with gap 316 (e.g., such that the one or more pumpbeams may enter from any or all of such side windows 320 and be incidentin substantially orthogonal fashion with the lasing gas), such as isshown in FIG. 6A, discussed below. In some cases, outer conduit 360 mayhave one or more end windows 322 disposed therein. In some embodiments,the end window(s) 322 may be disposed at throated portion 366 andaligned with gap 316 along the lasing axis (e.g., such that the DPALoutput beam may be directed out of a given end window 322 and the one ormore pump beams may enter from one or more of such end windows 322 andbe incident in substantially orthogonal fashion with the lasing gas). Insome embodiments, a given end window 322 may be oriented such that itsmajor surfaces (e.g., the surfaces through which the DPAL output beam isto pass) are substantially perpendicular (e.g., exactly or otherwisewithin a desired variance) to the lasing axis of the DPAL. However, theclaimed invention is not so limited, as in some other cases, a given endwindow 322 may be oriented/offset at an angle relative to the lasingaxis (e.g., at or near the Brewster's angle; etc.). Furthermore, aspreviously noted, some embodiments may include windows 320 and/or 322which are apertures/open spaces or optical mirrors, as desired for agiven target application or end use. Other suitable optical window320/322 configurations for outer conduit 360 will depend on a givenapplication and will be apparent in light of this disclosure.

It should be noted, however, that the claimed invention is not solimited. For instance, in some other embodiments, outer conduit 360 maybe configured such that a greater portion thereof (e.g., a larger areaof throated portion 366) is constructed from or otherwise includes adesired optical material, optical mirror, aperture, etc., and thusprovides a side window 320, an end window 322, etc., of largerdimensions. In some still other embodiments, outer conduit 360 itselfmay be constructed from a desired optical material and thus serve as agiven optical window (e.g., a side window 320, an end window 322, etc.).Numerous configurations will be apparent in light of this disclosure.

In accordance with an embodiment, inner conduit 310 may be designed tophysically separate the lasing gas flow, for example, from the barriergas co-flow. As such, it may be desirable to provide an inner conduit310, for example, having semi-rigid walls. To that end, and inaccordance with an embodiment, inner conduit 310 may be constructed, inpart or in whole, for example, with: (1) a metal (e.g., titanium,aluminum, etc.); (2) an alloy (e.g., stainless steel); (3) a ceramic;(4) an optical material, such as fused silica, sapphire, etc.; (5) ahigh-temperature polymer (e.g., polytetrafluoroethylene); (6) acombination of any of the aforementioned; and/or (7) any other suitablematerial, as will be apparent in light of this disclosure.

In some embodiments, inner conduit 310 may be generally configured witha contoured shape, much like outer conduit 360 discussed above. Forinstance, as can be seen from FIG. 5B, the nozzle portion 312 of innerconduit 310 may be provided with a generally converging shape, while thediffuser portion 314 thereof may be provided with a generally divergingshape. In accordance with an embodiment, a gap 316 may be provided orotherwise defined between the exit 312 a of nozzle portion 312 and theinlet 314 a of diffuser portion 314 such that the walls of inner conduit310 are interrupted or otherwise not continuous. The interior of innerconduit 310 may be configured to define, in part or in whole, the flowpath for the lasing gas flow. Also, as previously noted, the exterior ofthe walls of inner conduit 310 may help to define, in conjunction withthe interior of the walls of outer conduit 360, the flow path for thebarrier gas co-flow.

By virtue of such a configuration, inner conduit 310 may be configured,in general, to convert pressure to velocity (e.g., from the nozzleportion 312 to the gap 316) and then back to pressure again (e.g., fromthe gap 316 through the diffuser portion 314). The generally convergingshape of nozzle portion 312 may help to accelerate the lasing gas flow,whereas the generally diverging shape of diffuser portion 314 may helpto decelerate the lasing gas flow. In some instances, such aconfiguration may assist with heat removal from pumping cavity 10. Aswill be appreciated in light of this disclosure, it may be desirable toconfigure inner conduit 310 so as to: (1) minimize or otherwise reducepower losses which may result during the pressure/velocity conversionprocesses; and/or (2) accelerate and/or decelerate the lasing gas flowwith minimal or otherwise negligible loss of stagnation pressure.

In accordance with an embodiment, inner conduit 310 may be positionedwithin outer conduit 360 such that gap 316 is substantially aligned withone or more of the windows (e.g., flush side windows 320 and/or endwindows 322) which may be included at throated portion 366 of outerconduit 360. By virtue of such an arrangement, the one or more pumplasers may be permitted to pass through the outer housing 360 at flushside windows 320 and/or end windows 322 and through to the lasing gasflow in gap 316 with minimal or otherwise negligible interference frominner conduit 310 (e.g., by nozzle portion 312 and/or diffuser portion314). In some cases, gap 316 may have a length or other dimension in therange of about 1-10 mm or greater (e.g., about 2-5 mm; about 5-8 mm;etc.). However, the claimed invention is not so limited, as in someother example embodiments, gap 316 may have a length or other dimensionin the range of about 1-10 cm or greater (e.g., about 2-5 cm; about 5-8cm; etc.) Other suitable dimensions for gap 316 will depend on a givenapplication and will be apparent in light of this disclosure.

In accordance with an embodiment, nozzle portion 312 of inner conduit310 may be configured to deliver the lasing gas to gap 316. Thegenerally converging/tapering shape (e.g., in one or multipledimensions) of nozzle portion 312 may help to accelerate the lasing gasflow as it leaves exit 312 a to enter gap 316. In some cases, it may bedesirable to accelerate the lasing gas flow with minimal or otherwisenegligible loss of stagnation pressure. In some embodiments, the portionof the walls of inner conduit 310 which define the exit 312 a of nozzleportion 312 may decrease or otherwise taper in thickness (i.e., may beconfigured with knife-edges 313) as they progress towards gap 316, ascan be seen with reference to FIG. 6A, discussed below. It should benoted that while FIGS. 5A and 5B depict example embodiments in whichnozzle portion 312 and diffuser portion 314 taper in only one dimension,the claimed invention is not so limited. For instance, in some otherexample embodiments, one or more of nozzle portion 312 and/or diffuserportion 314 (e.g., and thus exit 312 a and/or inlet 314 a, respectively)may taper in two or more dimensions, as desired for a given targetapplication. In accordance with an embodiment, knife-edged walls 313 mayhelp to retain laminar flow by keeping the barrier gas co-flow and thelasing gas flow separated as they traverse gap 316.

As will be appreciated in light of this disclosure, it may be desirableto configure nozzle portion 312, for example: (1) to minimize orotherwise reduce the pump power; and/or (2) to maximize or otherwiseincrease the mass throughput for a given pump power. In some instances,it may be desirable to maximize or otherwise increase the dischargecoefficient of nozzle portion 312. In some embodiments, nozzle portion312 may be configured to provide a mass flow rate in the range of about1-100 g/s (e.g., about 1-10 g/s; about 10-80 g/s; etc.) or greater.Other suitable mass flow rates and/or configurations for nozzle portion312 will depend on a given application and will be apparent in light ofthis disclosure.

In accordance with an embodiment, nozzle portion 312 may be configuredwith any geometry desired for a given target application. In someinstances, adjustment may be made to the geometry of nozzle portion 312,for example, to produce different power output levels and/oroptical-to-optical efficiencies for a given DPAL system. In someembodiments, nozzle portion 312 may be configured with a geometry thatis substantially symmetrical. For instance, in one specific exampleembodiment, nozzle portion 312 may be provided with a substantiallyrectangular cross-sectional geometry (e.g., such as can be seen fromFIG. 5B). However, the claimed invention is not so limited, as in someother embodiments, the geometry of nozzle portion 312 may beasymmetrical. In some instances, it may be desirable to configure nozzleportion 312 such that: (1) the lasing gas flow leaving exit 312 aexperiences minimal or otherwise negligible frictional pressure losses;and/or (2) the lasing gas flow is a substantially parallel, planar jetupon leaving exit 312 a and entering pumping cavity 10. To that end, andin accordance with an embodiment, computational fluid dynamicssoftware/programming (e.g., ANSYS® Fluent® software) may be used, forexample, to determine what geometry of nozzle portion 312 may producethe desired results.

In accordance with an embodiment, nozzle portion 312 may be dimensionedas desired for a given target application. For instance, some examplesuitable aspect ratios (e.g., length-to-width ratios) for nozzle portion312 may include, but are not limited to: about 80-to-1 or less; about50-to-1 or less; about 20-to-1 or less; about 15-to-1 or less; about10-to-1 or less; about 5-to-1 or less; about 2-to-1 or less; etc. Thus,in the example context in which nozzle portion 312 has an aspect ratioof about 10-to-1, nozzle portion 312 may be dimensioned, for instance,with: a length of about 13 mm and a width of about 1.3 mm; a length ofabout 3 cm and a width of about 3 mm; a length of about 10 cm and awidth of about 1 cm; etc. However, the claimed invention is not solimited, as in some other example instances, greater and/or lesseraspect ratios may be utilized, in accordance with an embodiment. In oneexample case, nozzle portion 312 may be configured such that its exit312 a converges to a geometry that is about 1 mm wide and about 1 cmdeep. In some other example cases, nozzle portion 312 may be configuredsuch that its exit 312 a has at least one dimension in the range ofabout 1-10 mm (e.g., for use with a stable resonator configuration). Insome still other example cases, nozzle portion 312 may be configuredsuch that its exit 312 a has at least one dimension in the range ofabout 1-10 cm (e.g., for use with an unstable resonator configuration).Furthermore, in some cases, the cross-sectional area of nozzle portion312 may decrease (e.g., as it converges) by a factor of greater thantwo, greater than five, greater than ten, or greater than twenty goingin the direction of the lasing gas flow (e.g., towards the exit 312 a ofnozzle portion 312). Other suitable dimensions and/or aspect ratios fornozzle portion 312 will depend on a given application and will beapparent in light of this disclosure.

In accordance with an embodiment, diffuser portion 314 of inner conduit310 may be configured to recapture/collect the lasing gas flow after ithas traversed gap 316 and/or to recover the dynamic pressure of thelasing gas flow. The generally diverging shape of diffuser portion 314may help to expand the lasing gas, converting velocity to pressure andthus decelerating the lasing gas flow. In some cases, it may bedesirable to decelerate the lasing gas flow with minimal or otherwisenegligible loss of stagnation pressure while reducing turbulence. Thus,in some embodiments, the portion of the walls of inner conduit 310 whichdefine the inlet 314 a of diffuser portion 314 may be configured withblunted-edges 315 (e.g., as can be seen with reference to FIG. 6A,discussed below) which may help to separate the lasing gas flow from thebarrier gas co-flow as the lasing gas enters the inlet of diffuserportion 314. Downstream of diffuser portion 314, inner conduit 310 maybe configured as a gradually diverging channel, which may help toconvert the kinetic energy of the lasing gas into increased staticpressure with minimal or otherwise acceptable frictional losses.

In accordance with an embodiment, diffuser portion 314 may be configuredwith any geometry desired for a given target application. In someinstances, adjustment may be made to the geometry of diffuser portion314, for example, to produce different power output levels and/oroptical-to-optical efficiencies for a given DPAL system. In much thesame fashion as discussed above in the context of nozzle portion 312,diffuser portion 314 may be configured with a geometry that issubstantially symmetrical or asymmetrical, as desired. In someembodiments, diffuser portion 314 may be provided with a cross-sectionalgeometry which substantially matches that of nozzle portion 312 (e.g., asubstantially rectangular cross-section, such as can be seen from FIG.5B). In some instances, it may be desirable to configure diffuserportion 314 such that the lasing gas flow entering at inlet 314 aexperiences minimal (or otherwise negligible) frictional pressurelosses. To that end, and in accordance with an embodiment, computationalfluid dynamics software/programming (e.g., ANSYS® Fluent® software) maybe used, for example, to determine what geometry of diffuser portion 314may produce the desired results.

In accordance with an embodiment, diffuser portion 314 may bedimensioned as desired for a given target application. As will beappreciated in light of this disclosure, it may be desirable in someinstances to configure diffuser portion 314 with an aspect ratio and/ordimensions which substantially match those of nozzle portion 312. Forinstance, in one example embodiment, inlet 314 a of diffuser portion 314may be provided with a geometry that is about 1 mm wide and about 1 cmdeep to match exit 312 a of nozzle portion 312. However, the claimedinvention is not so limited, as in some other instances, diffuserportion 314 (e.g., inlet 314 a) may be provided with an aspect ratioand/or dimensions which are greater than those of nozzle portion 312(e.g., exit 312 a). Other suitable dimensions for diffuser portion 314will depend on a given application and will be apparent in light of thisdisclosure.

FIG. 6A is a partial cross-sectional view of a flush side windowassembly 300, configured in accordance with an embodiment of the presentinvention. As can be seen, flush side window assembly 300 may beconfigured to deliver two gas streams (e.g., a lasing gas flow via innerconduit 310 and a surrounding barrier gas co-flow via outer conduit 360)to the pumping cavity 10 and to recover those streams downstream of thepumping cavity 10. In the depicted example embodiment, the lasing gasmay enter gap 316 at the exit 312 a of nozzle portion 312. While in gap316, the lasing gas may pass through pumping region 396, where it may beoptically pumped, for example, by one or more pump beams providedthrough: (1) one or more flush side windows 320; (2) one or more endwindows 322; and/or (3) a combination of any of the aforementioned orother optical input/output window configuration (e.g., apertures,optical mirrors, etc.). Downstream of pumping region 396, the lasing gasmay be recaptured at the inlet 314 a of diffuser portion 314.

The co-flowing gas may be made to enter the pumping cavity 10 from theupstream portion 362 of outer conduit 360 outside of nozzle portion 312.By virtue of this configuration, the co-flowing gas may be made tocircumscribe/surround the lasing gas volume as it traverses gap 316,thus providing a centrally located planar/laminar jet of lasing gassurrounded by a co-flowing planar/laminar jet of a non-alkali barriergas. As will be appreciated in light of this disclosure, thethree-dimensional nature of the internal lasing gas flow and thesurrounding barrier gas co-flow generally can be likened, in someexample cases, to a rectangular box within a rectangular tube.Thereafter, the co-flowing gas may be recovered by the downstreamportion 364 of outer conduit 360 outside of diffuser portion 314.

As can be seen best from FIGS. 5B and 6A, flush side window assembly 300may be configured (e.g., by virtue of how outer conduit 360 and/or innerconduit 310 are configured) to provide a pumping region 396 having asubstantially rectangular box-like/cuboid geometry. In one specificexample embodiment, pumping region 396 may have a volume in the range ofabout 0.01-10 cm³ or greater (e.g., about 1 cm³ or less; about 3 cm³ orless; etc.). Other suitable geometries and/or dimensions for pumpingregion 396 will depend on a given application and will be apparent inlight of this disclosure.

In some cases, pumping region 396 may be made to reside substantially(e.g., entirely or otherwise within a given tolerance) within the TEM₀₀mode envelope 397 of the pumping cavity 10 (e.g., of a stable opticalresonator; of an unstable optical resonator; etc.). As will beappreciated in light of this disclosure, it may be desirable to ensurethat the pump region 396 is overlapped as much as possible (or to anotherwise desired degree) by the TEM₀₀ mode envelope 397 (discussedbelow) of the stable/unstable optical resonator and that the lasing gasstream is permitted to flow such that the one or more pump beamsintersect the lasing gas stream substantially (e.g., only or otherwisewithin a given tolerance) inside the TEM₀₀ mode envelope 397. In somecases, such a configuration may result in high geometrical DPALefficiency and thus help to maximize output beam power.

As previously noted, the lasing gas can be configured to have anapproximately rectangular stream geometry (e.g., by virtue of how nozzleportion 312 may be configured). In accordance with an embodiment, thisstream geometry can be substantially contained (e.g., fully or otherwisewithin a desired tolerance) within the DPAL's TEM₀₀ Gaussian output beam(e.g., implementing either a stable or unstable optical cavity). Thus,such a stream geometry may help to ensure that any pump-bleached alkalivapor atoms and/or ions lie within a confined volume which can have highgeometrical overlap (e.g., full or an otherwise desired amount) with theTEM₀₀ mode envelope 397 of the DPAL's Gaussian output beam. As will beappreciated in light of this disclosure, the higher the geometricoverlap between the DPAL TEM₀₀ Gaussian output beam and the rectangularpump-bleached alkali vapor region, the greater the suppression of ASElosses. In accordance with an embodiment, this may help to increaseoptical-to-optical efficiency and/or reduce the waste heat per watt ofthe DPAL output beam. In accordance with an embodiment, the presence ofthe co-flowing gas around the lasing gas flow may help to eliminateregions (e.g., at the flush side window 320) of alkali atoms and/or ionswhich otherwise might be pumped but not stimulated and thus lead to ASElosses which would lower optical-to-optical efficiency (as previouslydiscussed).

As will be appreciated in light of this disclosure, and in accordancewith an embodiment, adjustment to the dimensions of inner conduit 310(e.g., nozzle portion 312, diffuser portion 314, gap 316, etc.) and/orouter conduit 360 may be made to alter the dimensions of the lasing gasflow relative to the dimensions of the barrier gas co-flow and/or viceversa. In some cases, the net cross-sectional area of the lasing gasflow and the barrier gas co-flow may be approximately equal. In someinstances, the lasing gas flow may be dimensioned (e.g., by virtue of agiven configuration of inner conduit 312) such that it is less than orequal to the dimensions of the TEM₀₀ mode envelope 397. In some cases,the TEM₀₀ mode envelope 397 may have a width/diameter in the range ofabout 1-3 mm or greater. In some other cases, such as in the context ofan unstable resonator, the TEM₀₀ mode envelop 397 may have awidth/diameter in the range of about 3-30 mm or greater. As will befurther appreciated in light of this disclosure, and in accordance withan embodiment, adjustment to the dimensions of inner conduit 310 (e.g.,nozzle portion 312, diffuser portion 314, gap 316, etc.) and/or outerconduit 360 may be made, in part, to alter the gas flow of flush sidewindow assembly 300. Furthermore, and in accordance with an embodiment,the dimensions of inner conduit 310 (e.g., nozzle portion 312, diffuserportion 314, etc.) may be adjusted as desired to ensure that gap 316 issufficiently large to prevent or otherwise reduce incidence of innerconduit 310 with the one or more pump beams and/or DPAL output beam(e.g., by providing a suitable distance between TEM₀₀ envelope 397 andknife-edged walls 313 and/or blunt-edged walls 315).

FIG. 6B is a partial cross-sectional view of a flush side windowassembly 300, configured in accordance with an embodiment of the presentinvention. As can be seen, the co-flowing gas may be made to pass besidethe lasing gas flow, separated by a substantially rectangular perimeterof: (1) knife-edged walls 313 on the delivery side of the gap 316 (e.g.,provided by the exit of nozzle portion 312); and/or (2) blunt-edgedwalls 315 on the receiver side of the gap 316 (e.g., provided by theinlet of diffuser portion 314). In some cases, it may be desirable toconfigure nozzle portion 312 and/or diffuser portion 314 so as toprovide a boundary between the co-flowing gas and the lasing gas flowwhich results in minimal or otherwise negligible flow separation acrossthe gap 316 and/or reduces generation of small scale flow vortices.

As can be seen further, in some cases, diffusive mixing regions 398 mayresult alongside the lasing gas flow. In such diffusive mixing regions398, the lasing gas may intrude into the co-flowing gas during itstransit across gap 316. However, as generally depicted by thedashed-and-dotted lines in FIG. 6B, the lasing gas which enters theco-flowing gas may be carried away downstream without being incident tothe flush side windows 320. Thus, and in accordance with an embodiment,the co-flowing gas may: (1) function as a barrier layer between theamplifier gas and the flush side windows 320, preventing attack thereofby pump-bleached alkali atoms and/or ions; (2) carry away reactionproducts (if any) to minimize or otherwise reduce fouling of the flushside windows 320; and/or (3) carry away excess locally generated heatresulting from pumping of the lasing gas by the one or more pump beams.Other suitable uses of the co-flowing gas will depend on a givenapplication and will be apparent in light of this disclosure.

In some embodiments, the lasing gas and the co-flowing gas may enterpumping cavity 10 with different temperatures. For instance, in oneexample, the lasing gas passing through nozzle portion 312 may have atemperature in the range of about 100° C. or greater (e.g., about150-200° C.), while the co-flowing gas passing around nozzle portion 312may have a temperature in the range of about 100° C. or less (e.g.,about room temperature). Other suitable temperature ranges and/ordifferences for the lasing gas and/or the co-flowing gas will depend ona given application and will be apparent in light of this disclosure.

In accordance with an embodiment, the lasing gas and the co-flowing gasmay be permitted to physically interact, for example, only during thetransit across gap 316 (e.g., at the one or more diffusive mixingregions 398), which may be relatively brief in duration (e.g., in therange of about 5-10 μs or less). The brevity of this transit may help tominimize or otherwise reduce loss of the alkali vapor of the lasing gasinto the co-flowing gas. To ensure that the cross-mixing of the barriergas and the lasing gas is minimized (or otherwise negligible) during thetransit across gap 316, it may be desirable to ensure that: (1) thenozzle portion 312 is configured to match (e.g., approximately equal orotherwise within a given tolerance) the velocity of the co-flowing gasand the lasing gas flow; and/or (2) the diffuser portion 314 isconfigured for equal pressure recovery. Matching the pressure and/orvelocity of the co-flowing gas with that of the lasing gas flow (e.g.,to within a factor of less than 1.1, less than 1.3, less than 1.5, etc.,difference) may help, for instance: (1) to minimize eddy-current mixingof the two gas streams; (2) to reduce the presence of vorticalstructures; and/or (3) to reduce shear-flow turbulences. As will beappreciated in light of this disclosure, this may help to minimize orotherwise reduce flow-induced unsteadiness in pumping region 10, andthus may help to increase optical-to-optical efficiency, in accordancewith an embodiment. In some example cases, the lasing gas and co-flowinggas may be provided with a flow velocity in the range of about 50-1000m/s or less (e.g., about 700 m/s or less; about 780 m/s or less; about860 m/s or less; etc.). However, the claimed invention is not solimited, as in some other instances, the flow velocity may be greaterthan about 1000 m/s. Other suitable flow velocities for the lasing gasand/or co-flowing gas will depend on a given application and will beapparent in light of this disclosure.

As will be appreciated in light of this disclosure, and in accordancewith an embodiment, it may be desirable to provide a lasing gas flow andbarrier gas co-flow having a ratio of velocities which minimizes orotherwise reduces shear between the streams and thus minimizes orotherwise reduces the generation of small vortices which would causeundesirable mixing of the gases. However, as will be appreciatedfurther, the lasing gas and/or the co-flowing gas may be subjected toheating from any of a number of sources/mechanisms while traversing gap316, which may increase their velocities (and thus alter theratio/matching of their velocities) through gap 316. For instance, insome cases a portion of the one or more pump beams and/or the beamwithin the resonator cavity (e.g., the would-be DPAL output beam) may beabsorbed by outer conduit 360 and/or inner conduit 310 (e.g.,particularly in the region of knife-edged walls 313 of nozzle portion312 and blunt-edged walls 315 of diffuser portion 314), producinglocalized heating of those surfaces which, in turn, may transfer heat tothe lasing gas and/or co-flowing gas. Furthermore, in some cases, powerlosses due to fluorescence and/or ASE may result in illumination ofouter conduit 360 and/or inner conduit 310, particularly in the regionof knife-edged walls 313 and blunt-edged walls 315, producing localizedheating which may transfer heat to the lasing gas and/or co-flowing gas.Still further, collisional quenching of the pump level to the lasinglevel (e.g., of three-level alkali atoms, such as Rb) may result in afew percent power loss (e.g., about 2% for Rb), in some instances, whichmay be deposited exclusively or otherwise primarily into the lasing gas,thereby heating the lasing gas. Heating from these and/or othersources/mechanisms may result, for instance, in thermal expansion andconsequent acceleration of the gases. Therefore, velocity matching ofthe lasing gas flow and barrier gas co-flow may depend, in part, onaccounting for the spatial distribution of heat deposition into the twogas streams.

As can be seen, for example, from the embodiments illustrated in FIGS.5A-5B and 6A-6B, inner conduit 310 can be configured such that thecross-sectional area of the lasing gas flow entering gap 316 from nozzleportion 312 is about equal to the cross-sectional area of the lasing gasflow leaving gap 316 to enter diffuser portion 314. As can further beseen, outer conduit 360 and inner conduit 310 can be configured suchthat the void/region defined there between (e.g., which guides thebarrier gas co-flow) has an approximately equal cross-section on bothsides of gap 316 (e.g., leaving upstream portion 362 and enteringdownstream portion 364). As will be appreciated in light of thisdisclosure, and in accordance with an embodiment, such a configurationmay be desirable, for example, in some cases in which heating of theco-flowing gas around the region of gap 316 is comparable with heatingof the lasing gas passing through gap 316.

However, there may be instances in which the heating of the co-flowinggas is not comparable to that of the lasing gas. For instance, if powerloss mechanisms which deposit power into the co-flowing gas dominate,then the co-flowing gas may absorb more heat and thus expand more thanthe lasing gas. Conversely, if power loss mechanisms that deposit powerinto the lasing gas dominate, then the lasing gas may absorb more heatand thus expand more than the co-flowing gas. Therefore, and inaccordance with an embodiment, the spacing between blunt-edged walls 315of diffuser portion 314 relative to the spacing between knife-edgedwalls 313 of nozzle portion 312 may be adjusted, for example, to improvematching of the velocities of the lasing gas and the co-flowing gasthrough/around gap 316.

FIG. 7A is a side perspective view of a multiple barrier side windowassembly 300′ configured in accordance with an embodiment of the presentinvention. As can be seen, assembly 300′ may be configured in much thesame way as assembly 300 (discussed above), with an example differencebeing that the walls defining outer conduit 360 may be interrupted orotherwise not continuous (e.g., much like previously noted with respectto inner conduit 310) and the one or more side windows 320 and/or endwindows 322 may be omitted from such outer conduit 360. Even with suchan interruption, outer conduit 360 may be configured to provide thedesired barrier gas co-flow, as previously discussed, which may help toconfine the lasing gas volume. As can further be seen, an externalhousing 380 may be included and configured so as to surround orotherwise enclose outer conduit 360 (e.g., upstream portion 362 anddownstream portion 364) and inner conduit 310 (e.g., nozzle portion 312and diffuser portion 314). As will be appreciated in light of thisdisclosure, and in accordance with an embodiment, housing 380 may beconstructed using any of the example materials previously discussed withreference to outer conduit 360 and inner conduit 310. In some cases,housing 380 may have disposed therein or otherwise be configured toprovide one or more optical windows (e.g., side windows 320, end windows322, etc.), which may be aligned with gap 316, as previously described.

In accordance with an embodiment, housing 380 may be configured toprovide a bleed flow of a non-alkali barrier gas (e.g., much like thatdescribed above in the context of FIGS. 3 and 4B) which may assist inisolating the one or more side windows 320 and/or end windows 322 ofhousing 380 from the lasing gas flowing through inner conduit 310. Theinterstitial space between the walls of outer conduit 360 and the wallsof housing 380 generally may define the flow path for the barrier gasbleed flow. Thus, and in accordance with an embodiment, assembly 300′may be configured to provide a given DPAL system with multiplecontainment/barrier layers of alkali-free gas(es), which can be used toaid in preserving any of the optical surfaces (e.g., windows, coatings,etc.) implemented therewith from chemical attack by the alkali vapor ofthe lasing gas and/or soot buildup. Other suitable configurations forhousing 380 and/or assembly 300′ will depend on a given application andwill be apparent in light of this disclosure.

FIG. 7B is a partial cross-sectional view of a multiple barrier sidewindow assembly 300′, configured in accordance with an embodiment of thepresent invention. As can be seen, the barrier gas bleed flow throughhousing 380 may be made to pass outside of the barrier gas co-flow,separated by the outer walls of upstream portion 362 and downstreamportion 364. In accordance with an embodiment, it may be desirable insome instances to allow a small amount of the non-alkali gas of thebleed flow (e.g., in the range of less than or equal to about 0.1% to1%) to mix into the barrier gas co-flow and to be swept downstreamthrough inlet 314 a of diffuser portion 314, while the remainder of thebleed flow migrates downstream between outer conduit 360′ and housing380. In accordance with an embodiment, the barrier gas bleed flow ofassembly 300′ may: (1) function as a barrier layer between the barriergas co-flow and the flush side windows 320 of housing 380; (2) carryaway reaction products (if any) to minimize or otherwise reduce foulingof the flush side windows 320; and/or (3) carry away excess locallygenerated heat. Other suitable uses of the barrier gas bleed flow inconjunction with the barrier gas co-flow will depend on a givenapplication and will be apparent in light of this disclosure.

In some embodiments, a given flush side window assembly 300/300′ may beoperatively coupled (e.g., fluidly coupled) with a gas circulationsystem 800, discussed below with reference to FIG. 9, which isconfigured: (1) to circulate the lasing gas through inner conduit 310;(2) to circulate the barrier gas through outer conduit 360; and/or (3)to circulate the bleed flow gas through housing 380.

Supporting Gas Circulation Systems

FIG. 8A is a schematic view of an example implementation of a lasing gascirculation system 700 a configured in accordance with an embodiment ofthe present invention. As can be seen, lasing gas circulation system 700a may be utilized, for example, in the context of a diffusive/convectiveend window assembly 100. Lasing gas circulation system 700 a may beconfigured to drive a lasing gas circulation loop (represented by thebolded line in the figure), which provides the lasing gas (e.g.,buffer/carrier gas including alkali vapor) to pumping cavity 10. As willbe appreciated in light of this disclosure, lasing gas circulationsystem 700 a may include additional, fewer, and/or different elements orcomponents from those here described, and the claimed invention is notintended to be limited to any particular system configurations, but canbe used with numerous configurations in numerous applications.

In accordance with an embodiment, the gas to be used as the gaseousbarrier layer in an end window assembly 100 may be derived from a smallbleed flow from the lasing gas circulation loop which is subsequentlydirected to a trap and/or filtration component 710. Trap and/orfiltration component 710 may be configured, in some embodiments, toremove the alkali vapor from the lasing gas. Also, trap and/orfiltration component 710 may be configured to provide the separatedbuffer/carrier gas for downstream use (e.g., to be used as a gaseousbarrier layer in end window assembly 100). Trap and/or filtrationcomponent 710 may include one or more filters, for example, forfiltering out any unwanted substances (e.g., particles, ions, vapors,etc.) which may be produced during the lasing process, therebymaintaining the cleanliness of the barrier gas. The buffer/carrier gasmay be delivered to the one or more injectors 130 of end window assembly100 and can form a gaseous barrier layer, as previously discussed (e.g.,in the context of FIG. 3). Other suitable implementations of lasing gascirculation system 700 a will depend on a given application and will beapparent in light of this disclosure.

FIG. 8B is a schematic view of an example implementation of a lasing gascirculation system 700 b configured in accordance with an embodiment ofthe present invention. As can be seen, lasing gas circulation system 700b may be utilized, for example, in the context of a recessed side windowassembly 200. As will be appreciated in light of this disclosure, and inaccordance with an embodiment, lasing gas circulation system 700 b maybe configured in much the same way as the lasing gas circulation system700 a discussed above in the context of FIG. 8A. Here, however, thebuffer/carrier gas may be delivered to the one or more injectors 230 ofrecessed side window assembly 200 and can form a gaseous barrier layer,as previously discussed (e.g., in the context of FIG. 4B). Othersuitable implementations of lasing gas circulation system 700 b willdepend on a given application and will be apparent in light of thisdisclosure.

FIG. 9 is a schematic view of an example implementation of a gascirculation system 800 configured in accordance with an embodiment ofthe present invention. As can be seen, gas circulation system 800 may beutilized, for example, in the context of a flush side window assembly300. Gas circulation system 800 may be configured to drive two separatecirculation loops: (1) a lasing gas circulation loop (represented by thesolid bold line in the figure); and (2) a barrier gas circulation loop(represented by the dashed bold line in the figure). As will beappreciated in light of this disclosure, gas circulation system 800 mayinclude additional, fewer, and/or different elements or components fromthose here described, and the claimed invention is not intended to belimited to any particular system configurations, but can be used withnumerous configurations in numerous applications. For instance, as willbe appreciated in light of this disclosure, and in accordance with anembodiment, gas circulation system 800 optionally can be configured toprovide a barrier gas bleed flow for an assembly 300′, as discussedabove with reference to FIGS. 7A-7B.

The lasing gas circulation loop may be operatively coupled with innerconduit 310 and configured: (1) to deliver the lasing gas to the pumpingcavity 10, for example, at nozzle portion 312; and (2) to recapture thelasing gas after its transit through pumping cavity 10, for example, atdiffuser portion 314. The barrier gas circulation loop may beoperatively coupled with outer conduit 360 and configured: (1) todeliver the co-flowing gas to the pumping cavity 10, for example, aroundnozzle portion 312 of inner conduit 310; and (2) to recapture theco-flowing gas, for example, around diffuser portion 314 of innerconduit 310. In some instances, the barrier gas circulation loop alsomay be configured to deliver and recapture the bleed flow gas, forexample, outside of an outer conduit 360 of an assembly 300′ (aspreviously discussed).

During its transit along the barrier gas circulation loop, the gas thatis used as the co-flowing barrier layer (and/or bleed flow layer, ifdesired) may be directed through a trap and/or filtration component 810,in accordance with an embodiment. Trap and/or filtration component 810may be configured, for example, to trap or otherwise remove any alkalivapor which may have mixed/leaked from the lasing gas flow into thebarrier gas co-flow during the transit between nozzle portion 312 anddiffuser portion 314. Also, trap and/or filtration component 810 may beconfigured to provide the resultant non-alkali buffer/carrier gas fordownstream use (e.g., to be used as the co-flowing gaseous barrier layerfor flush side window assembly 300 and/or as the bleed flow barrierlayer for flush side window assembly 300′). In some cases, trap and/orfiltration component 810 may include one or more filters, for example,for filtering out any unwanted substances (e.g., particles, ions,vapors, etc.) which may be produced during the lasing process, therebymaintaining the cleanliness of the non-alkali barrier gas and/oralkali-saturated lasing gas. In some instances, the buffer/carrier gasmay be directed through outer conduit 360 to provide the protectiveco-flow around the lasing gas jet/stream provided by inner conduit 310and can form a co-flowing gaseous barrier layer, as previously discussed(e.g., in the context of FIGS. 5A-5B and 6A-6B). In some furtherinstances, the buffer/carrier gas may be directed through housing 380 toprovide a protective bleed flow around the co-flow of outer conduit 360to form one or more bleed flow barrier layers, as previously discussed(e.g., in the context of FIGS. 7A-7B). Other suitable implementations ofgas circulation system 800 will depend on a given application and willbe apparent in light of this disclosure.

The foregoing description of the embodiments of the invention has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many modifications and variations are possible in light ofthis disclosure. It is intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto.

What is claimed is:
 1. A diode-pumped alkali laser (DPAL) comprising: anoptical surface; a flow of a lasing gas, wherein the lasing gas includesalkali vapor; and a flow of a first non-alkali gas flowing between theoptical surface and the flow of the lasing gas.
 2. The DPAL of claim 1,wherein the flow of the first non-alkali gas forms a protective layeralong the optical surface to prevent or minimize at least one ofchemical attack thereto and/or soot buildup thereon.
 3. The DPAL ofclaim 1, wherein the first non-alkali gas comprises at least one of aninert gas, a noble gas, a hydrocarbon, a fluorocarbon, and/or acombination of any thereof.
 4. The DPAL of claim 1 further comprising aflow of a second non-alkali gas flowing between the optical surface andthe flow of the first non-alkali gas.
 5. The DPAL of claim 4, whereinthe flow of the second non-alkali gas forms a protective layer along theoptical surface to prevent or minimize at least one of chemical attackthereto and/or soot buildup thereon.
 6. The DPAL of claim 4, wherein thesecond non-alkali gas comprises at least one of an inert gas, a noblegas, a hydrocarbon, a fluorocarbon, and/or a combination of any thereof.7. The DPAL of claim 1, wherein the alkali vapor comprises at least oneof rubidium (Rb), cesium (Cs), and/or potassium (K).
 8. The DPAL ofclaim 1, wherein the optical surface comprises an optical window or amirror.
 9. The DPAL of claim 8, wherein the optical surface furthercomprises an optical coating.